PTMEG is a commodity in the chemical industry, used mainly in the manufacture of polyurethanes and polyesters. Functioning as a chain segment, PTMEG imparts resilience, flexibility, and softness to these materials. Generally, PTMEG is manufactured by way of cationic ring-opening polymerization of tetrahydrofuran (THF) in the presence of a suitable cationic initiator such as fluorosulfonic acid. Such polymerization of THF was first observed by Meerwein et al. in the late 1930's and is discussed in German Patent No. 74 1,476.
The molecular weight of PTMEG significantly influences the physical properties of both the PTMEG and the products derived from it. For example, PTMEG of relatively low molecular weight is a sticky, viscous oil at room temperature while, at higher molecular weights, it is thicker and more waxy. As is true of all polymeric compounds, the molecular weight of PTMEG may be expressed as either a "number-average" or a "weight-average". The number average molecular weight (Mn) is obtained by dividing the weight of a sample by the number of molecules of which it is composed. The weight-average molecular weight (Mw), on the other hand, is defined as the ratio of the sum of the mathematical products obtained by multiplying each molecular weight present in the sample by its corresponding fractional weight and the total sample weight. In practice, Mn and Mw are normally determined via such methods as gel permeation chromatography (GPC). Since a typical sample of PTMEG is composed of molecules of different degrees of polymerization, a distribution of molecular weights exists and as a result, values of Mn and Mw are not equivalent. The ratio Mw/Mn, referred to as the polydispersity, is indicative of the breadth of distribution of molecular weights for a given sample of polymer.
Although PTMEG may be produced with molecular weights in the millions, the most useful commercial varieties are those with number average molecular weights in the range between about 250 and 3500. The most common method of producing PTMEG with a Mn in this range is by polymerizing THF in the presence of fluorosulfonic acid (FSA). That method has several disadvantages however. For example, the FSA used in the polymerization cannot be recovered for reuse and the disposal of the toxic and corrosive spent acids, sulfuric acid and hydrofluoric acid, produced as a by-product present a serious environmental problem. In addition, the PTMEG product may contain some small but significant number of fluorine end groups.
More recently, processes for the manufacture of PTMEG have been developed which eliminate the FSA disposal problem. In U.S. Pat. No. 4,120,903, for example, Pruckmayr et al. describe a process in which PTMEG is produced by polymerizing THF using a polymeric catalyst which contains sulfonic acid groups. Representative of such catalysts is NAFION.RTM. perfluorosulfonic acid resin, a product of E. I. du Pont de Nemours and Company. The Mn of the PTMEG produced according to that invention was about 10,000 in the example cited. On the other hand, Heinsohn et al. in U.S. Pat. No. 4,163,115, disclose a process for manufacturing esters of PTMEG through polymerization of THF in a medium containing an acylium ion precursor such as acetic anhydride, a polymeric catalyst which contains sulfonic acid groups, and optionally, a carboxylic acid such as acetic acid. Manipulating the ratio of carboxylic acid to acylium ion precursor affords enhanced control of molecular weight of the ester. According to that invention, ester end-capped PTMEG with Mn in the rang of 660 to 3,000 may be produced.
In instances where the molecular weight of the PTMEG produced by such processes is greater than that desired, it would be useful to have a process by which the molecular weight of PTMEG could be reduced to one within the acceptable range. In one such method (U.S. Pat. No. 3,925,484, M. C. Baker), high molecular weight PTMEG is treated with a strong sulfonic acid ion-exchange resin as catalyst to partially reverse the polymerization of THF, thereby converting some of the high molecular weight PTMEG to THF monomer and yielding PTMEG with a narrow molecular weight distribution. The number average molecular weight of the polymer tends to increase initially because of the high number of low molecular weight chains which are more easily depolymerized. If the duration of Baker's treatment were prolonged, depolymerization of the high molecular weight chains would take place to an increasing extent so that ultimately essentially all of the initial PTMEG would be converted to THF.